A semiconductor diode laser is a monocrystalline pn junction device. In one form of such a device, the pn junction is in a plane disposed in an active region between two parallel rectangular faces of a monocrystalline semiconductor body. Two mutually parallel reflective faces that are perpendicular to the pn junction form a laser cavity. Laser action is produced by applying a forward bias voltage across the pn junction. The forward bias injects electrons and holes across the pn junction. These electrons and holes recombine in the active region and give rise to stimulated emission of radiation in the form of photons. Above a given level of electron injection, called the threshold current (I.sub.th), emitted radiation is collected and amplified in the active region. The amplified radiation exits the active region parallel the pn junction as a monochromatic beam.
The threshold current, I.sub.th, is an increasing function of operating temperature, so that, as temperature increases, more charge carriers must be injected across the pn junction to cause laser action. Thus, to create semiconductor diode lasers that operate at higher temperatures, the threshold current must be reduced and/or the charge carriers must be injected into the active region more efficiently.
One problem in reducing threshold current is that electrons and holes can be injected into the active region without resulting in stimulated emission therein. For example, they can escape outside the active region to adjacent portions of the semiconductor body, where they recombine without contributing to laser emission. Even assuming that the stimulated emission takes place, the photons produced in the active region can escape from the active region by radiation in a direction that is not parallel to the pn junction. In addition, it is possible for electrons to disappear within the active region without producing the desired emission of radiation, such as by combining with holes at crystal defects. All such losses increase the threshold current.
The design of a semiconductor diode laser can be changed to resist the escape of injected electrons ano holes and stimulated photons from the active region by sandwiching the active region between two contiguous layers of monocrystalline semiconductive material having a larger energy band gap and a lower index of refraction than the active region. The active region thus becomes an optical cavity. The difference in energy band gaps retains the charge carriers while the changed index of refraction confines the photons in the optical cavity. The optical cavity, and as a practical matter, the two contiguous layers must be of a very high monocrystalline quality. This requires that these layers and the active region be closely matched not only in crystal structure but also in crystal lattice size. Moreover, one of the sandwiching layers must be doped to n-type conductivity and the other to p-type conductivity. Such a structure is referred to herein as a double heterojunction semiconductor diode laser.
The efficiency of injecting charge carriers (electrons, for example) into the optical cavity is reduced by the escape of sufficiently energetic such carriers from the optical cavity to the sandwiching layer doped to have a conductivity type opposite to the carrier type. It is therefore desirable to create a double heterojunction semiconductor diode laser with increased injection current efficiency by reducing electron leakage current into the p-type side of the pn junction or pull leakage current into the p-type side of the pn junction.